Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-RAM). STT-RAM utilizes magnetic junctions written at least in part by a current driven through the magnetic junction. For example, FIG. 1 depicts a conventional magnetic tunneling junction (MTJ) 10 as it may be used in a conventional STT-RAM. The conventional MTJ 10 typically resides on a bottom contact 11, uses conventional seed layer(s) 12 and includes a conventional antiferromagnetic (AFM) layer 14, a conventional first pinned, or reference layer 16, a conventional tunneling barrier layer 18 and a conventional free layer 20. Also shown is top contact 30.
The conventional reference layer 16 and the conventional free layer 20 are magnetic. The conventional free layer 20 has a changeable magnetization 21. The magnetization 17 of the conventional reference layer 16 is fixed, or pinned, in a particular direction, typically by an exchange-bias interaction with the AFM layer 14. The magnetizations 25 and 17 of the conventional free layer 20 and pinned layer 16, respectively, may be in-plane as shown or perpendicular to the plane. The dual MTJ 10 may have enhanced spin torque if the reference layer 16 and 26 are fixed in the dual state (magnetic moments 17 and 25 of reference layers 16 and 26 antiparallel). However, a dual MTJ 10 in the dual state may have reduced magnetoresistance. In contrast, if the reference layers 16 and 26 are fixed in the antidual state (magnetic moments 17 and 25 of reference layers 16 and 26 parallel) the dual MTJ 10 may have enhanced magnetoresistance. In the antidual configuration, however, the spin-transfer torque contributions from two reference layers 16 and 26 counteract each other. As a result, spin transfer based switching may require a larger write current for the antidual state.
To switch the magnetization 21 of the conventional free layer 20, a current is driven perpendicular to plane (in the z-direction). The current carriers are spin polarized and exert a torque on the magnetization 21 of the conventional free layer 20 as the current carriers pass through the conventional free layer 20. The spin transfer torque on the magnetic moment 21 of the conventional free layer 20 is initially small when the magnetic moment 21 is parallel to the easy axis (the stable state). When a sufficient current is driven from the top contact 30 to the bottom contact 11, the magnetization 21 of the conventional free layer 20 may switch to be parallel to the magnetization 17 of the conventional reference layer 16. When a sufficient current is driven from the bottom contact 11 to the top contact 30, the magnetization 21 of the free layer may switch to be antiparallel to that of the reference layer 16. The differences in magnetic configurations correspond to different magnetoresistances and thus different logical states (e.g. a logical “0” and a logical “1”) of the conventional MTJ 10.
In applications such as STT-RAM, the memory cell including the conventional magnetic junction 10 is selected. Typically, this is accomplished by configuring each memory cell to include both the conventional magnetic junction 10 and a selection transistor (not shown in FIG. 1). When the transistor is turned on, for example by a voltage applied to the transistor's gate, current can be driven through the conventional dual magnetic junction 10 in the CPP configuration. This current can be a read current or a write current for STT writing. Data may thus be written to or read from the conventional MTJ 10.
Although the conventional dual MTJ 10 may be written using spin transfer and used in an STT-RAM, there are drawbacks. For example, very high currents may be required to achieve switching of the conventional free layer 20 at a sufficiently low write error rate. These currents may require a larger selection transistor and/or may damage the conventional dual magnetic junction 10. Further, to obtain such a high spin transfer torque, the reference layers 16 and 26 have their magnetic moments 17 and 25 in the antidual state (fixed in opposite directions). When in this state, there is a cancellation of magnetoresistance during a read operation, which lowers the read signal. Such a reduction in signal is undesirable.
Accordingly, what is needed is a method and system that may improve the performance of the spin transfer torque based memories. The method and system described herein address such a need.